Last month I attended a NASA test of an RS-25 rocket engine. Even weeks later, I think about the event in awe and wonder. The A-1 test stand to which the engine was mounted is impossibly large. The B-2 test stand, which will eventually test an array of four RS-25s, is larger even still. The engine test itself was like experiencing what I can only describe as a peaceful apocalypse. The sound and reverberations and billowing clouds had the all the terrible power and spectacle of what I imagine the end of the world to be like, but there, at NASA Stennis Space Center, the force was put toward the cause of peace and the betterment of humankind. I'm not given to mawkishness about such things, but all I could think the whole time was: If we can do this, we can do anything.
The only other time I've ever felt that way was visiting the Great Wall of China—another impossible human achievement of breathtaking size, scope, and engineering genius. I feel fortunate to have seen two such wonders in a single lifetime.

I wrote a little about the test for Mental Floss:

It was magnificent. The engine was like an inverted volcano. White clouds billowed forth at 13 times the speed of sound, blasting so forcefully that even its component water vapor seemed confused and alarmed. The sound was like a sustained, rolling thunder that you could feel in your teeth, and its timbre dominated even your pulse. The experience was truly awesome in force and effect. The power and fury of the test was terrifying—and yet the RS-25 is perhaps the most peaceful product of the space age thus far. It's not a weapon of war. It powers no ballistic missiles, nuclear or otherwise. It exists only for exploration and the betterment of humankind.

I've embedded a video below. It doesn't come close to capturing the thing. The clouds you see billowing from the engine test stand are made of water vapor. The RS-25 burns liquid oxygen and liquid hydrogen; the engine leaves behind an Earth as clean as it found it.